Forms and Balance of Nitrogen and Phosphorus in Cage Culture Waters in Guangdong Province, China

In order to approach the characteristics of nitrogen and phosphorus pollution caused by cage culture and the balance of nitrogen and phosphorus during the process of cage culture, a monitoring was conducted in Daya Bay of Guangdong Province, China from April 2002 to Jane 2003. The results show that the concentrations of total nitrogen （TN） in the waters at the sites with five and ten years of cage culture history are 1.8 and 2.3 times of that at control site respectively. Ammonium （NH3-N） is the main form of nitrogen in spring while nitrate （NO~） in winter. The concentrations of TN, total dissolved nitrogen （TDN） and dissolved organic nitrogen （DON） are highest in autumn. The concentration of phosphorus increases with the increasing of the culturing time, among which phosphate （ PO^-34） increases most obviously. The concentrations of total phosphorus （TP） and total dissolved phosphorus （TDP） are highest in autumn. The nitrogen and phosphorus are accumulated significantly in the sediment of cage culture area. The model of N balance in the cage culture area： bait （70.62%） ＋ fry （0.28%） ＋ input by tide （14.8%） ＋ release from sediment 04.3%） = harvest of adult fish （12.07%） ＋ deposition into sediment （28.75%） ＋ output by tide （56.18%） ＋ others （3.00%）. The model of P balance： bait （83.11%） ＋ fry （0.17%） ＋ input by tide （12.23%） ＋ release from sediment （4.49%） = harvest of adult fish （8.43%） ＋ deposition into sediment （48.59%） ＋ output by tide （41.94%） ＋ others （1.04%）. In one fish growth year, the contents of nitrogen and phosphorus in harvest of adult fish are only 17.0% and 10.1% of the contents of nitrogen and phosphorus in fish bait and fry, wherein 83% of nitrogen and more than 89% of phosphorus in fish bait became marine pollutants.

Aquaculture Environmental Capacity Research of Daiqu Fishery of Zhoushan: The Impact of Cage Culture on Sediment Quality

Impact of LaMmichthys crocea cage culture in Daiqu fishery （Zhoushan, Zhejiang Province） on the chemistry of the sediment was inves- tigated. Sediment samples, in different depths and from three stations at various distances from the cage, were collected, for analyzing total organic matter, total organic carbon, total nitrogen and phosphorus. The results showed a significant increase in all analyzed parameters in station 1, at 5 m from the cage. The difference between reference station （600 m from the cage） and station 2 at 95 m to the cage was insignificant, indicating lo- calized impact of cage farming to the vicinity of cage. The analyzed parameters in various depths did not show a significant difference. The values of analyzed parameters in the perimeter of the cage and their differences with reference stations showed small magnitude and localized impact on the chemistry of sediment. It might be because of deep water and moderate velocity of water current in this fjord. The magnitude of impact may dif- fer durina the summer when biomass and feedina rate would be at the maximum level.

Determine the Optimal Density of Nile Tilapia (<i>Oreochromis niloticus</i>) Fingerlings Cultured in Floating Cages

The objective of the study was to determine the optimal density of fingerlings of the Nile tilapia (Oreochromis niloticus) culture in floating cages with respect to growth performance and to know the effect of temperature on growth during the winter period. Tilapia fingerlings (mean weight 8.5 ± 0.36 g) were stocked at densities of 120, 180 and 360 fish/m3 for 10 weeks. During culture, fish were fed a diet contains 35% protein made of peanut seed, cottonseed, wheat bran, and Sorghum. The results showed that the increase in fish density reduced the final weight of the fish. The percentage of the fish weight increased and reached 111% ± 1.45% in the lowest fish density (120 fish/m3), 79% ± 1.87% in the medium density (240 fish/m3), and 63% ± 2.03% of high fish density (360 fish/m3), therefore the daily growth rate was better in the lowest density. The rate of food conversion ratio was also better in the lowest density with 2.17 ± 0.33, followed by the medium density with 2.79 ± 0.28 and then the highest density with 3.09 ± 0.21. However, the total production in the cage increased with increasing fish density, and the return to the cost of food was better with higher fish density. According to various performance evaluation criteria, it can be concluded that the medium density 240 fish/m3 is the best for growing Nile tilapia fingerlings in floating cages. It was observed that the growth rates of the three treatments were low due to the decrease in water temperature in most of the study period, which may indicate the useless of culture in cages in the winter season.

Comparison for ecological economic performance of Chinese sea perch(Lateolabrax Maculatus)under different aquaculture systems

Chinese sea perch(Lateolabrax Maculatus)is one of the main marine fish culture species in China,its green development has received increasing attention and its ecological economic study is rare.Based on field survey,this paper takes water pollution tax of China as conversion indicator for aquaculture eutrophication to simulate aquaculture waste fee for further analyzing the ecological impact of Chinese sea perch’s aquaculture.For data analysis,data envelopment analysis model has been used.The comparison for Chinese sea perch’s economic performance and ecological economic performance under its main aquaculture systems has been done.The results show that:1.The simulated waste fee has no significant impact for the comparison.2.Ranking for ecological economic performance’s ecological economic,technical and scale efficiency is as follow:pond farming,offshore cage culture and cage culture.3.Offshore cage culture is the only system presenting positive increase for ecological economic performance,which makes its aquaculture activity more sustainable than other systems.4.The key point of green development is to maximize the ecological economic efficiency of main aquaculture systems.Thus,offshore cage culture is the most sustainable aquaculture system among China’s current marine fish culture systems,and other systems need innovation for achieving the green development of China’s aquaculture.